Perforated screens for the pressure screen printing of carpets and the like, wherein the screens are constituted as drums with patterns of perforation through which the dyestuff is forced under pressure onto the carpet, have been made heretofore by galvanic techniques.
So-called printing screens have generally been produced heretofore by coating a metal cylinder or substrate with a photosensitive lacquer layer, drying the lacquer layer and exposing that layer through a film having a pattern corresponding to the desired perforation pattern or raster. After the film has been exposed,, the photosensitive layer is developed so that the portions of the photosensitive material surrounding an array of dots corresponding to the perofration pattern are removed to expose the substrate drum surface. Metal is then galvanically deposited, i.e. electroplated, onto the drum around the dots of the retained photosensitive material so that, when the deposited metal body or screen is removed from the drum, perforations remain corresponding to the locations of the dots.
In practice it has been found that the openings in the screen are reduced, with reference to the dot pattern of the film and to the dots of photosensitive material left upon the substrate drum, because of encrustation resulting from the electroplating process. In other words, electroplated metal does not deposit strictly perpendicularly onto the substrate drum but also grows toward the centers of the openings or perforations which are intended to be defined by the dots of photosensitive material retained on the substrate drum.
It has also been found that the extent of encrustation, i.e. the extent to which the electroplated metal grows toward the center of the opening, increases with the thickness of the deposited metal and, therefore, the thickness of the printing screen. Thus it is only possible to ensure a cross section of the openings in the screen which approximately corresponds to the cross section of the dots around which the metal is electrodeposited when the screen formed from this metal is relatively thin.
In pressure printing, i.e. where the dyestuff is to be forced through the dot-pattern screen onto, for example, a carpet, the thinness of the screen is determined by its ability to withstand the elevated printing pressures which are employed. In other words, the pressure-printing requirements limit the degree to which the thickness of the screen can be minimized. It has been the practice heretofore, therefore, to accept a reduction in the cross section and size of the perforations, resulting from the boundary growth of metal during electroplating, when relatively thick screens are to be made, i.e. when the thickness of the screen is sufficient for pressure-screen printing as is necessary in the printing of carpet materials with dyestuffs.